Switch mode power supplies are used in a broad range of electronic applications, such as, for example, telecommunication systems, computers, television equipment, video equipment, etc., where it is important to accurately regulate a power supply output over a wide range of loads. A significant advantage of switch mode power supplies over other types of regulated power supplies (e.g., linear power supplies) is that switch mode power supplies are smaller, weigh less, consume less power, and are highly efficient compared to many other types of regulated power supplies.
In certain applications, a switch mode power supply must act as a current source with over-voltage protection. A conventional voltage source typically shuts down during a short circuit, which leaves only an open circuit as the minimum load condition. As the load on a current source moves towards an open circuit, the power supply attempts to increase the supply voltage in order to reach a predetermined current set point. Consequently, a voltage limit is placed on the supply, so that if the load opens up completely, the supply will be limited at an upper voltage. Otherwise, the output voltage would increase until components begin to fail.
A conventional technique that is used to provide current mode regulation and the voltage limit function in a switch mode power supply operating as a current source is to generate two feedback signals. One of the two feedback signals is controlled by the current set point for the supply, and the second feedback signal is controlled by the voltage limit for the supply. The two feedback signals are OR'd together to produce a signal that indicates the loading of the supply. This OR'd signal (e.g., referred to as a compensation signal) is then used to control the duty cycle of the oscillator and the switching in the switch mode power supply. An example of this feedback signal technique that can be used for controlling the duty cycle of the switching of a power supply is illustrated in FIG. 1.
FIG. 1 depicts an existing feedback circuit 100 that can be used to provide a compensation signal for a switch mode power supply. In typical applications, circuit 100 is commonly used for power supply feedback in a switch mode power supply operating as a current source. As shown, circuit 100 includes a first input connection 102, which is connected to measure the output voltage (e.g., VOUT) at the power supply load (not shown). A (load) voltage sensed at connection 102 is developed across resistor 104 and applied to a first input of an amplifier stage (e.g., operational amplifier) 106. A second input of amplifier stage 106 is connected to a reference voltage source 108 (e.g., shunt regulator), which is selected to provide a desired voltage limit for the supply. Resistor 110 is a feedback resistor for amplifier stage 106. Thus, amplifier stage 106 functions essentially as an error amplifier, with the output of amplifier stage 106 increasing as the (scaled) output voltage drops below the reference voltage 108, or decreasing as the (scaled) output voltage rises above the reference voltage 108. The output from amplifier stage 106 is coupled through a diode 112 to a pullup resistor 114. The resistor 114 is connected to a circuit supply 116 and works together with diodes 112 and 130 to “diode-OR” the outputs from the amplifiers together. The resulting signal forms the compensation signal output connect 118.
Circuit 100 also includes a second input connection 120, which is connected to measure the output current (e.g., IOUT) delivered to the load (not shown), by reading the voltage developed across a resistor (not shown) in series with the load. The voltage proportional to the (load) current sensed at connection 120 is passed though resistor 122 and applied to a first input of a second amplifier stage (e.g., operational amplifier) 124. A second input of amplifier stage 124 is connected to a reference voltage source 126 (e.g., shunt regulator), which is selected to provide a desired current set point (e.g., 60 ma) for the supply. Notably, although two reference voltage sources (108, 126) are shown in FIG. 1, these two reference voltage sources are typically implemented with one device. Resistor 128 is a feedback resistor for amplifier stage 124. Thus, amplifier stage 124 functions as a second error amplifier, and the output of amplifier stage 124 is set by the difference between the desired reference 126 and the sensed output current 120. The output from amplifier stage 124 is coupled through a diode 130 to a pullup resistor 114. The pullup resistor 114 is connected to a circuit supply 116 and works together with diodes 112 and 130 to “diode-OR” the outputs from the amplifiers 106 and 124 together. The resulting signal forms the compensation signal output connect 118. The diodes 112 and 130 function to OR the output signals from amplifiers 106 and 124, and enables whichever sensed parameter is the highest above its desired reference to control the power supply by way of the feedback circuit 100 this allows the supply to operate with stability in both current regulating and voltage limiting modes.
Notwithstanding the distinct advantages of the existing power supply feedback signal technique that can be used in a switch mode power supply, a significant problem that still exists with such switch mode power supplies is that they are generally more expensive than other regulated power supplies (particularly at the lower power levels), because of the relatively high number and cost of components in the switch mode power supplies and relative complexity of the circuitry involved. Consequently, the bulkier, less accurate, and more inefficient power supplies are often used in cost-sensitive applications. Therefore, it would be advantageous to have a switch mode power supply that can operate efficiently and accurately over a very wide range of loads, and is also much smaller and less expensive than the existing switch mode power supplies. As described in detail below, the present invention provides such a much smaller, less expensive switch mode power supply.